The information superhighway will primarily comprise optical fibers for the foreseeable future because of the enormous bandwidth that each optical fiber provides. For example, an optical fiber exhibits relatively low loss over the wavelength region 820-1600 nanometers. This particular region provides a bandwidth of about 180,000 GHz which means that a single optical fiber can potentially carry 45 billion voice channels (4 kHz each) or 30 million television channels (6 MHz each). And while these numbers represent upper limits that are not practical to attain, they provide a compelling reason for communication carriers to use optical transmission.
However, in order to fully utilize this information superhighway, there needs to be convenient equipment for inserting and removing individual optical channels, or groups of channels, at multiple intermediate locations along an optical fiber path. Equipment that performs this function is not surprisingly known as an optical add/drop multiplexer (ADM), and it generally comprises a demultiplexer connected in series with a multiplexer. Between the demultiplexer and the multiplexer, another optical device is used to add and/or remove selected channels.
However, because the ADMs are cascaded in large optical networks, the transmission characteristic of each channel that undergoes demultiplexing and multiplexing is narrowed--a phenomenon that can be readily understood by recognizing that cascading these passive devices is equivalent to multiplying their individual transmission functions together. For example, assuming that the transmission characteristic of one particular channel of an ADM has a normalized magnitude of 1.0 at the center of its passband and 0.8 at its edges, then the transmission characteristic of that particular channel through two ADMs is still 1.0 at the center, but is now 0.8.times.0.8=0.64 at the edges. The resulting passband thus has a more pronounced central peak, which effectively reduces the usable bandwidth of each passband. Accordingly, the number of ADMs that can be cascaded is also reduced.
Optical multiplexing and demultiplexing is frequently accomplished via a pair of star couplers that are interconnected by an optical grating (i.e., a number of parallel waveguides--each differing in length with respect to its nearest neighbor by a predetermined fixed amount). Examples of such interconnected star couplers, known as Dense Wave Division Multiplexers (DWDMs), are shown in U.S. Pat. Nos. 5,002,350 and 5,136,671 and 5,412,744. In one direction of optical transmission, the DWDM can be used as a multiplexer wherein a plurality of separate and distinct wavelengths (.lambda..sub.1, .lambda..sub.2. . . .lambda..sub.n, ) are launched into different input ports of one star coupler and emerge on a single output port of the other star coupler. In the other direction of optical transmission, the DWDM can be used as a demultiplexer wherein a plurality of different wavelengths are launched into a single port of one star coupler and emerge on multiple ports of the other star coupler according to their particular wavelengths. An ADM can therefore be built using two DWDMs connected back-to-back.
Techniques are known for making the passband of a DWDM relatively wide and flat. For example, U.S. Pat. No. 5,412,744 achieves wide and flat passbands by coupling adjacent waveguides at the input or at the output of the DWDM. Further, application Ser. No. 08/682,453 was filed on Jul. 17, 1996 by the present inventor, and it achieves wide and flat passbands by installing multiple power splitters in the same DWDM. Nevertheless, when such DWDMs are cascaded in an ADM configuration, it is desirable to further improve the transmission characteristics of the individual passbands.